[willamette-fcst] Silverton Hills AM Forecast.doc

Thu Aug 9 08:55:34 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU AUG 9, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 1 p.m. until 3 p.m. with a 50 acre limit.

Propane flaming is allowed from noon until 5 p.m.

WEATHER DISCUSSION:

A broad upper-level ridge of high pressure remains anchored over the Southwestern United States with a southwesterly flow aloft over Oregon.  At the surface, onshore gradients have relaxed and turned more northerly.  The Salem sounding this morning showed northeasterly winds extending from just above the surface up through 4000 feet. That has resulted in a less expansive marine layer across western Oregon this morning.  However, a weak upper-level disturbance, centered off the southern Oregon coast, is spreading considerable clouds across the region. It is forecast to slowly move across the state today; keeping skies at least partly cloudy.

The air aloft has warmed several degrees in the past 24 hours, so high temperatures should be close to normal this afternoon, in spite of some clouds.  The warming aloft will make for lower mixing heights this afternoon, compared to Wednesday.  However, gradient stacking will be less of an issue for burning.  The northeasterly transport winds should turn northerly by late-morning and could possibly turn enough onshore, late this afternoon, to create a limited burning opportunity.

(Salem Airport on Wednesday: High 77°F; Rainfall .00")

TODAY'S FORECAST:

Mostly cloudy this morning. Partly cloudy this afternoon.  A little warmer.

Salem's high temperature today will be near 82 degrees (normal is 83).

Relative humidity drops to 50% by 2 p.m. and to near 40% by 5 p.m.

Surface winds:  N 5-10 mph this morning; NNW 7-12 mph this afternoon.

Transport winds: N 10 mph; becoming NNW 10-16 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 3800 feet by 5 p.m.

Salem's sunset tonight: 8:26 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 67       76      81      74

     Relative Humidity:           65%      48%     40%     50%

     Surface Wind Direction:       N        N     NNW      NW

     Surface Wind Speed:           6        8       9      12

     Transport Wind Direction:     N        N     NNW      NW

     Transport Wind Speed:        10       12      15      17

     Mixing Height:             2200     3000    3800    2000

     Ventilation Index:           22       36      57      34

EXTENDED DISCUSSION:

An upper-level trough will move across western Oregon tonight and slightly reinforce the marine layer.  That should produce morning clouds, on Friday, with afternoon sunshine.  There is a slight chance of a late-day limited burning window, but morning gradient stacking will need to be overcome and northerly transport winds will need to turn onshore.  Little change to the weather pattern is forecast through early next week, with continued possible limited late-day burning opportunities.

The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text

Notes:

     1.  Mixing height, as used here, is the lowest height at which the

         potential temperature exceeds the potential temperature at the

         surface.  As a practical matter it is the approximate height to

         which a smoke plume will rise assuming good ignition, dry fuels, and

         winds less than about 15mph.

     2.  Transport winds are a layer average through the mixing height,

         weighted slightly toward the winds at the top of the layer.

     3.  Ventilation Index is the height of the mixing layer times

         the transport wind speed divided by 1000.

     4.  Surface wind direction is the general expected wind direction.

         At a specific point surface winds are highly dependent on local

         terrain conditions.

This forecast is provided under an agreement between the Oregon Department of

Agriculture and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons

ODF Meteorologist
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